Explanation of anomalous binding kinetics with a high yield immobilized enzyme system

The activities of glucose oxidase (β-d-glucose:oxygen 1-oxidoreductase, EC 1.1.3.4) and catalase (hydrogen-peroxide:hydrogen-peroxide oxidoreductase, EC 1.11.1.6) from commercial preparations do not give typical adsorption curves upon immobilization on non-porous polyethylenimine-coated glass microbeads. The cause of this effect with glucose oxidase was investigated. Protein binding exhibited a rectangular hyperbolic adsorption isotherm, approaching saturation at high concentrations, however, enzyme activities did not. The isotherm for activities exhibited a maxima which corresponded to less than 50% saturation with regard to total protein adsorption. The enzyme preparation was found to contain small quantities of several low molecular weight impurities as judged by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. These impurities apparently compete with glucose oxidase for binding. When large excesses of protein are added to beads, the binding of impurities becomes significant and the amount of enzyme activity per unit of bead is reduced.     

Chemical and catalytic properties of the peroxisomal acyl-coenzyme A oxidase from Candida tropicalis

The peroxisomal acyl-CoA oxidase has been purified from extracts of the yeast Candida tropicalis grown with alkanes as the principal energy source. The enzyme has a molecular weight of 552,000 and a subunit molecular weight of 72,100. Using an experimentally determined molar extinction coefficient for the enzyme-bound flavin, a minimum molecular weight of 146,700 was determined. Based on these data, the oxidase contains eight perhaps identical subunits and four equivalents of FAD. No other β-oxidation enzyme activities are detected in purified preparations of the oxidase. The oxidase flavin does not react with sulfite to form an N(5) flavin-sulfite complex. Photochemical reduction of the oxidase flavin yields a red semiquinone; however, the yield of semiquinone is strongly pH dependent. The yield of semiquinone is significantly reduced below pH 7.5. The flavin semiquinone can be further reduced to the hydroquinone. The behavior of the oxidase flavin during photoreduction and its reactivity toward sulfite are interpreted to reflect the interaction in the N(1)-C(2)O region of the flavin with a group on the protein which acts as a hydrogen-bond acceptor. Like the acyl-CoA dehydrogenases which catalyze the same transformation of acyl-CoA substrates, the oxidase is inactivated by the acetylenic substrate analog, 3-octynoyl-CoA, which acts as an active site-directed inhibitor.     

Liver monoamine oxidase in the obese-hyperglycaemic (ob/ob) mouse.

1. The specific activity of monoamine oxidase was found to be greater in liver mitochondria from ob/ob mice than from lean mice. The activities of marker enzymes were similar in both tissues. 2. Experiments with various substrates (5-hydroxytryptamine, benzylamine and tyramine) and inhibitors (clorgyline and deprenyl) indicated that, unlike rat liver mitochondria, mouse liver mitochondria contain a predominance of the B-form of monoamine oxidase. 3. The Km values for lean and ob/ob mice were the same for any given substrate and were in the increasing order 5-hydroxytryptamine less than tyramine less than benzylamine. Vmax. was approximately 50% greater in obese than in lean mice. 4. Extraction of liver mitochondria with acetone/water or acetone/water/NH3 to remove lipids decreased the enzyme activity relatively more in obese- than in lean-mice preparations, but residual activity was the same in both preparations.     

Spectroscopic studies on the photoreaction of choline oxidase, a flavoprotein, with covalently bound flavin

Formation of the anionic flavosemiquinone was observed spectrophotometrically during the anaerobic photo-irradiation of Alcaligenes sp. choline oxidase in the presence of EDTA. Further irradiation slowly converted the semiquinone form into the fully reduced state. The presence of a catalytic amount of riboflavin greatly enhances the photoreduction rate not only to the semiquinone state but also to the fully reduced state. This semiquinone species has low reactivity toward the substrate, choline or betaine aldehyde, as well as toward oxygen. This low reactivity toward oxygen is unique to the semiquinone form of a flavoprotein oxidase. The oxidized enzyme forms a complex with betaine, the product of the enzymatic reaction of choline oxidase. The dissociation constant for this complex was found to be 17 mM by spectroscopic titration. Anaerobic photo-irradiation of the enzyme with a saturating amount of betaine in the absence of EDTA produces, with no detectable semiquinone formation, an absorption spectrum which resembles (but significantly differs from) that of the fully reduced form. This species was found to comprise two flavin species. One of them is rapidly oxidized to the oxidized form by oxygen and is thus assigned as the fully reduced state. The other is converted slowly to the oxidized form upon aerobic standing in the dark. We tentatively assigned this latter species as a C(4a)-adduct. Formaldehyde was detected as a product of this photoreaction. The amount of formaldehyde formed coincided with that of the fully reduced enzyme. On the basis of the results obtained we propose a mechanism of the photoreaction of the enzyme in the presence of betaine where a C(4a)-adduct and the fully reduced enzyme via an N(5)-adduct are formed. Betaine also affects the dithionite reduction. In the dithionite reduction of the oxidized enzyme, the semiquinone species is an intermediate in the conversion of the oxidized to the fully reduced form, while the reduction of the oxidized enzyme-betaine complex with dithionite produces the fully reduced form without any significant formation of the semiquinone species.     

Iron-sulfur components of succinate dehydrogenase: stoichiometry and kinetic behavior in activated preparations.

Extensively or completely activated preparations of beef heart succinate dehydrogenase have been investigated by electron paramagnetic resonance (EPR) techniques at 6 to 97 K. Reductive titrations with dithionite and rapid kinetic studies were performed with various types of soluble and membrane-bound preparations of the enzyme. The following components were detected and their behavior analyzed: a free radical, presumably arising from the covalently bound flavin on reduction, two iron-sulfur centers of the ferredoxin type, the signals of which appear on reduction, and a highpotential iron-sulfur component, detectable in the oxidized state. The high-potential component was only detected in complex II and inner-membrane preparations. This component and one of the ferredoxin-type centers were present in amounts close to stoichiometric with the flavin and were reduced by substrate. The other ferredoxin-type center was present in amounts between 0.1 and 0.5 times that of the flavin and was reduced only by dithionite. Of the components reduced by succinate, however, only a fraction (up to 50% of the high-potential iron-sulfur center and 40-60% of the ferredoxin-type iron-sulfur center) was reduced within the turnover time of the enzymes; In complex II not more than about 10% of the flavin appeared in the semiquinone form at any time. Soluble, purified preparations behaved similarly except that the high-potential component was nearly or completely absent and extensive accumulation of the free radical occurred (up to 70 to 80% of the flavin) in titration and kinetic experiments. No significant difference was observed between the rates of semiquinone formation and the reduction of the ferredoxin-type or high-potential centers by the substrate. Also no qualitative differences in the properties studied in this work became apparent between prepatations containing 4 or 8 iron atoms, respectively.     

Early changes in mitochondrial monoamine oxidase activity of rat liver during 2-acetylaminofluorene feeding

The activities of mitochondrial type A and B monoamine oxidase were determined in the liver of rats fed a diet containing 2-acetylaminofluorene (AAF). Three days after the initiation of AAF-feeding, there was a significant decrease of type B monoamine oxidase activity without affect on type A enzyme. The decreased activity of type B monoamine oxidase, which reached a minimum after three weeks, was sustained for as long as AAF-feeding was continued. Sex-related difference in response to AAF was seen in the rat with respect to the onset and the intensity of the decreased type B monoamine oxidase activity, male rats being more sensitive to the carcinogen than female rats. In contrast to the in vivo effect, AAF showed a potent inhibitory effect on type A monoamine oxidase, rather than on type B enzyme, when added in vitro. The pI₅₀ values were estimated to be 7.5 against type A monoamine oxidase and 4.1 against type B enzyme, respectively. The in vitro inhibition of both types of monoamine oxidase by AAF was competitive. The K_i values for AAF were calculated to be 9.51 · 10^?9 M for type A monoamine oxidase and 1.30 · 10^?5 M for type B enzyme, respectively. In accordance with the potent inhibitory effect of AAF on type A monoamine oxidase in vitro, a single administration of the carcinogen, at a dose of 50 mg/kg, resulted in a marked and temporal decrease of the enzyme activity in the mitochondria of male rat liver. Recovery of the decreased type B monoamine oxidase activity was slow, and the enzyme activity did not return to control levels, even if rats were fed the basal diet for 2 or 4 weeks after the cessation of AAF-feeding.     

Immunological Studies of Human Monoamine Oxidases

Antisera have been raised against monoamine oxidase preparations from human placenta and platelets. These antisera have been employed to characterize membrane-bound enzyme from a variety of human sources including liver, heart, and brain. The comparisons were based on a displacement radioimmunoassay system with soluble placental monoamine oxidase, previously labelled specifically with [3H]pargyline, as antigen. All forms of enzyme investigated demonstrated immunological cross reaction; however, the placental enzyme appeared to possess determinants not exhibited by the enzyme from the platelets or other tissues examined.     

Fluorescein mercuric acetate specifically displaces zinc from cytochrome oxidase

Treatment of beef heart cytochrome oxidase with fluorescein mercuric acetate (FMA) was found to specifically displace zinc from the enzyme and inhibit the steady-state activity in a parallel fashion. The native cytochrome oxidase preparation contained 2.3 Cu: 2.0 Fe: 1.1 Zn: 0.9 Mg. Addition of 2 equivalents of FMA inhibited the activity by 50% and displaced 60% of the zinc from the enzyme, but did not affect the copper, iron or magnesium content. The pre-steady-state reduction of cytochrome oxidase by ferrocytochrome c was not affected by the FMA treatment, in contrast to the inhibition of steady state activity. These results suggest a possible structural or functional role for zinc in cytochrome oxidase.     

One-electron reduction of D-amino acid oxidase. Kinetics of conversion from the red semiquinone to the blue semiquinone

The reduction of D-amino acid oxidase (DAAO) by hydrated electrons (eaq-) has been studied in the absence and presence of benzoate by pulse radiolysis. The eaq-did not reduce the flavin moiety in DAAO and reacted with the amino acid residues in the protein. In the presence of benzoate, eaq- first reacted with benzoate to yield benzoate anion radical. Subsequently, the benzoate anion radical transferred an electron to the complex of DAAO-benzoate to form the red semiquinone of the enzyme with a second-order rate constant of 1.2 X 10(9) M-1 s-1 at pH 8.3. After the first phase of the reduction, conversion of the red semiquinone to the blue semiquinone was observed in the presence of high concentration of benzoate. This process obeyed first-order kinetics, and the rate increased with an increase of the concentration of benzoate. In addition, the rate was found to be identical with that of the formation of the complex between benzoate and the red semiquinone of DAAO as measured by a stopped-flow method. This suggests that bound benzoate dissociates after the reduction of the benzoate-DAAO complex by benzoate anion radical and that free benzoate subsequently recombines with the red semiquinone of the enzyme to form the blue semiquinone.     

On preparative separation of brain mitochondrial monoamine oxidases.

A method was developed for solubilization from bovine brain stem mitochondrial fraction of monoamine oxidases deminating biogenic amines. Preparative separation of the monoamine oxidases, possessing different substrate specificities, was achieved by column chromatography on a biospecific adsorbent AH-Sepharose 4 B. The enzyme preparations thus obtained did not contain any detectable by disc-electrophoresis of isoelectrofocusing in polyacrylamide gels proteins which were devoid of the monoamine oxidase activity.     

On Preparative Separation of Brain Mitochondrial Monoamine Oxidases

A method was developed for solubilization from- bovine brain stem mitochondrial fraction of monoamine oxidases deminating biogenic amines. Preparative separation of the monoamine oxidases, possessing different substrate specificities, was achieved by column chromatography on a biospecific adsorbent AH-Sepharose 4 B. The enzyme preparations thus obtained did not contain any detectable by disc-electrophoresis or isoelectrofocusing in polyacrylamide gels proteins which were devoid of the monoamine oxidase activity.     

Effect of Latent Iron Deficiency on 5-Hydroxytryptamine Metabolism in Rat Brain

Eight weeks of latent iron deficiency in weaned rats maintained on an experimental low iron content diet (18-20 mg/kg) did not significantly alter the packed cell volume and hemoglobin concentration; however, the hepatic and brain nonheme iron contents decreased by 66% and 21% (p less than 0.001), respectively. The tryptophan concentration decreased by 31% and 34% in liver and brain, respectively, in rats on experimental diet (p less than 0.01). The brain 5-hydroxytryptamine and 5-hydroxyindoleacetic acid contents were reduced by 21% and 23% (p less than 0.01 and p less than 0.02), respectively. However, in the brain, weight, protein, DNA, and the activities of monoamine oxidase, aldehyde dehydrogenase, and liver tryptophan oxygenase were found to remain unaltered. When rehabilitated with a diet containing 390 mg/kg iron, rats previously maintained on the experimental diet for 2 weeks showed partial recovery in tryptophan levels both in liver and brain. However, brain 5-hydroxytryptamine and 5-hydroxyindoleacetic acid levels remained unaltered. The hepatic iron content improved without any change in brain iron content. The latent iron deficiency produced significant alterations in the metabolism of 5-hydroxytryptamine and brain iron content that could not be recovered 2 weeks after the iron rehabilitation.     

Effects of reversing the protein positive charge in the proximity of the flavin N(1) locus of choline oxidase

A protein positive charge near the flavin N(1) locus is a distinguishing feature of most flavoprotein oxidases, with mechanistic implications for the modulation of flavin reactivity. A recent study showed that in the active site of choline oxidase the protein positive charge is provided by His(466). Here, we have reversed the charge by substitution with aspartate (CHO-H466D) and, for the first time, characterized a flavoprotein oxidase with a negative charge near the flavin N(1) locus. CHO-H466D formed a stable complex with choline but lost the ability to oxidize the substrate. In contrast to the wild-type enzyme, which binds FAD covalently in a 1:1 ratio, CHO-H466D contained approximately 0.3 FAD per protein, of which 75% was not covalently bound to the enzyme. Anaerobic reduction of CHO-H466D resulted in the formation of a neutral hydroquinone, with no stabilization of the flavin semiquinone; in contrast, the anionic semiquinone and hydroquinone species were observed with the wild type and a H466A variant of the enzyme. The midpoint reduction potential for the oxidized-reduced couple in CHO-H466D was approximately 160 mV lower than that of the wild-type enzyme. Finally, CHO-H466D lost the ability to form complexes with glycine betaine or sulfite. Thus, with a reversal of the protein charge near the FAD N(1) locus, choline oxidase lost the ability to stabilize negative charges in the active site, irrespective of whether they develop on the flavin or are borne on ligands, resulting in defective flavinylation of the protein, the decreased electrophilicity of the flavin, and the consequent loss of catalytic activity.     

Significance of multiple forms of brain monoamine oxidase in situ as probed by electron spin resonance.

Spin-labeled hydroxyamphetamine, a competitive reversible inhibitor of brain monoamine oxidase, has been shown to be useful as an electron spin resonance (ESR) probe of the microenvironment of the active sites of the possible monoamine oxidase multiple forms. The ESR spectrum of spin-labeled hydroxyamphetamine was strongly quenched upon binding to the enzyme. The conformation of the active site of rat brain monoamine oxidase existing in various physical states, i.e. monoamine oxidase in situ (intact brain mitochondria), crude solubilized monoamine oxidase (MAOS) and isolated monoamine oxidase fractions (MAOa and MAOb) were critically and systematically examined. Nonlinear least squares regression analyses have been used to fit the binding data (obtained at room temperature with varying spin-labeled hydroxyamphetamine concentrations) to three groups of independent noninteracting ligand-binding models. A Gibbs-Helmholtz relationship was applied to the interpretation of the measured apparent association constant K as a function of temperature ranging from 4-50 degrees with increments of 2 degreesmfrom the extracted intensive parameters, k (intrinsic association constant) and deltaF (intrinsic free energy), as well as the apparent heat, deltaH, it was clear that the microenvironment of the binding sites existing in the more purified enzyme fractions MAOa and MAOb were similar to those found in the crude solubilized enzyme. More importantly, they correlated well with the conformation of the sites characterized in situ. The data suggested that the microenvironment of this multienzyme system was unperturbed in spite of the treatment due to the isolation process. In terms of the composition of binding sites, MAOa appeared to be heterogeneous while MAOb appeared to be more homogeneous. Since the isolated fractions MAOa and MAOb possessed marked different substrate specificities, these observations directly implied that monoamine oxidase multiple forms do exist in situ. The extracted extensive parameters, n (specific binding activity, nanomoles/mg of protein), as well as the measured characteristic transition temperatures, indicated that the relative abundance of the sites which directly affected substrate specificities was indeed altered. The consistency of the characteristic transition temperatures of 21 degrees and 38 degrees for the case of intact membrane preparations was particularly significant. A tenable hypothesis is that the manipulation in the composition of the monoamine oxidase binding forms through intimate lipid-protein interactions, which has been amply demonstrated in many biomembrane systems to be functionally important might be the underlying regulatory mechanism in vivo.     

L-amino-acid oxidase from the Malayan pit viper Calloselasma rhodostoma. Comparative sequence analysis and characterization of active and inactive forms of the enzyme.

Here we report the cDNA-deduced amino-acid sequence of L-amino-acid oxidase (LAAO) from the Malayan pit viper Calloselasma rhodostoma, which shows 83% identity to LAAOs from the Eastern and Western diamondback rattlesnake (Crotalus adamanteus and Crotalus atrox, respectively). Phylogenetic comparison of the FAD-dependent ophidian LAAOs to FAD-dependent oxidases such as monoamine oxidases, D-amino-acid oxidases and tryptophan 2-monooxygenases reveals only distant relationships. Nevertheless, all LAAOs share a highly conserved dinucleotide-binding fold with monoamine oxidases, tryptophan 2-monooxygenases and various other proteins that also may have a requirement for FAD. In order to characterize Ca. rhodostoma LAAO biochemically, the enzyme was purified from snake venom to apparent homogeneity. It was found that the enzyme undergoes inactivation by either freezing or increasing the pH to above neutrality. Both inactivation processes are fully reversible and are associated with changes in the UV/visible range of the flavin absorbance spectrum. In addition, the spectral characteristics of the freeze-and pH-induced inactivated enzyme are the same, indicating that the flavin environments are similar in the two inactive conformational forms. Monovalent anions, such as Cl(-), prevent pH-induced inactivation. LAAO exhibits typical flavoprotein oxidase properties, such as thermodynamic stabilization of the red flavin semiquinone radical and formation of a sulfite adduct. The latter complex as well as the complex with the competitive substrate inhibitor, anthranilate, were only formed with the active form of the enzyme indicating diminished accessibility of the flavin binding site in the inactive form(s) of the enzyme.     

Cytochrome c reduction by semiquinone radicals can be indirectly inhibited by superoxide dismutase

The inhibition by superoxide dismutase of cytochrome c reduction by a range of semiquinone radicals has been studied. The semiquinones were produced from the parent quinones by reduction with xanthine and xanthine oxidase. Most of the quinones studied were favored over O₂ as the enzyme substrate, and in air as well as N₂, semiquinone radicals rather than superoxide were produced and they caused the cytochrome c reduction. With all but one of the quinones (benzoquinone), cytochrome c reduction in air was inhibited by superoxide dismutase, but the amount of enzyme required for inhibition was up to 100 times greater than that required to inhibit reduction by superoxide. It was highest for the quinones with the highest redox potential. These results demonstrate how superoxide dismutase can inhibit cytochrome c reduction by species other than superoxide. They can be explained by the dismutase displacing the equilibrium: semiquinone + O₂ ? quinone + O₂^? to the right, thereby allowing the forward reaction to out-compete other reactions of the semiquinone. The implication from these findings that superoxide dismutase-inhibitable reduction of cytochrome c may not be a specific test for superoxide production is discussed.     

Inhibition of monoamine oxidase by substituted hydrazines

1. The initial rate of inhibition of monoamine oxidase by phenethylhydrazine was shown to be similar, in pH-dependence and kinetic properties, to the oxidation of that compound by monoamine oxidase. 2. The time-course of irreversible inhibition of monoamine oxidase by phenethylhydrazine lags behind that of reversible inhibition. 3. Hydralzine was shown to be a reversible competitive inhibitor of monoamine oxidase, but phenylhydrazine is an irreversible inhibitor. Inhibition by the latter compound is not affected by the absence of oxygen, and the presence of substrate exerts no protective action. 4. Hydrazine does not inhibit monoamine oxidase unless a substrate and oxygen are present. 5. Phenethylidenehydrazine was found to be a time-dependent inhibitor of monoamine oxidase and the rate of inhibition was hindered by increasing oxygen concentration. 6. A mechanism for the inhibition of the enzyme by phenethylhydrazine is proposed in which the product of oxidation of this compound is a potent reversible inhibitor and an irreversible inhibitor of the enzyme. A computer simulation of such a mechanism predicts time-courses of inhibition that are in reasonable agreement with those observed experimentally.     

Kinetic mechanism of monoamine oxidase A

Steady-state kinetic data for monoamine oxidase A in crude extracts suggest an exclusively ping-pong mechanism, in contrast to those for monoamine oxidase B, which indicate alternate mechanisms involving either a binary or ternary complex. In this study, with use of purified monoamine oxidase A, steady-state data for the inhibition by D-amphetamine of the oxidation of primary amines indicate the possibility of a ternary complex mechanism for monoamine oxidase A also. Stopped-flow studies demonstrate that the rate of reoxidation of reduced enzyme is enhanced by substrates but not by the product, 1-methyl-4-phenylpyridinium. Thus, for the A enzyme, the ternary complex with substrate, but not product, is reoxidized at a faster rate than the free, reduced enzyme. For both the A and B forms of monoamine oxidase, the mechanism is determined by competition between alternate pathways on the basis of the relative rate constants and dissociation constants.     

Effect of 1-methyl-4-phenylpyridinium ion (MPP+) on catecholamine levels and activity of related enzymes in clonal rat pheochromocytoma PC12h cells

1-Methyl-4-phenylpyridinium ion (MPP+), a metabolite of a neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, was found to reduce dopamine (DA) level and the activity of enzymes related to its metabolism in clonal rat pheochromocytoma PC12h cells. After 6 days' culture in the presence of 1 mM and 100 microM MPP+, DA content in PC12h cells was reduced markedly, but with MPP+ at concentrations lower than 10 microM, DA levels in the cells did not change. The amounts of 3,4-dihydrophenylacetic acid (DOPAC), a metabolite of DA were reduced markedly in culture medium and in PC12h cells cultured with MPP+ at concentrations higher than 1 microM. MPP+ was found to reduce the enzyme activity of tyrosine hydroxylase (TH), monoamine oxidase (MAO) and aromatic L-aminoacid decarboxylase (AADC). In the presence of MPP+ at concentrations higher than 10 microM, reduction of TH activity in the cells was more pronounced than reduction of cell protein or of the activity of a non-specific enzyme, beta-galactosidase. With 1 mM and 100 microM MPP+, MAO activity was reduced to about 30% of that in control cells. Reduction was observed with MPP+ at concentrations higher than 1 microM. AADC was the most sensitive to MPP+ and its activity was reduced markedly in the cells cultured with 100 nM MPP+. These results indicate that MPP+ inhibits not only the biosynthesis of catecholamines, but also the enzyme participating in their catabolism in cells, and may thus perturb catecholamine levels in the brain.     

ESR analysis of the FAD in bovine liver monoamine oxidase

Two hydrazine spin labels, 1-oxyl-2,2,5,5-tetramethylpyrroline-3-carbonyl ethyl hydrazine and 1-oxyl-2,2,6,6-tetramethylpiperidino-4-hydrazine, were synthesized as probes of the FAD binding site of monoamine oxidase. The reporter nitroxide moiety showed an ESR spectrum classified as partially immobilized which is indicative of FAD near the surface of the enzyme. Attempts to pick up flavin semiquinone or free radical intermediates during substrate oxidation with the spin traps 5,5-dimethyl-1-pyrroline-1-oxidase and phenyl-t-butylnitrone were not successful.